858 



THE SENSE ORGANS AND SKIN OF THE HORSE 



temporal (Angulus oculi medialis, lateralis). The lateral angle is rounded when the 

 eye is open, but the medial angle is narrowed and produced to form a 3-shaped 

 bay or recess, termed the lacrimal lake (Lacus lacrimalis) . In this there is a rounded 

 pigmented prominence kno\\'n as the lacrimal caruncle (Caruncula lacrimalis) ; it 

 is about the size of a small pea, and is covered Avith modified skin, connected ^vith 

 that of the medial commissure, from which project a number of hairs provided with 

 sebaceous glands. The lids unite on either side and form the commissures, medial 



and lateral (Commissura palpebrarum 

 medialis, lateralis). The anterior sur- 

 face of the lids (Facies anterior palpe- 

 brarum) is convex and is covered with 

 very short hair. A considerable num- 

 ber of tactile hairs are scattered over 

 the lower part of the lower lid, but on 

 the upper lid they are very scanty. 

 The infrapalpebral depression (Sulcus 

 infrapalpebralis) indicates somewhat 

 indistinctly the limit of the lower lid. 

 -The upper lid is marked by two fur- 

 rows when raised. The posterior sur- 

 face (Facies posterior palpebrarum) is 

 adapted to the free surface of the eye- 

 ball and is covered by the palpebral 

 conjunctiva. The free border of the 

 lid is smooth and usually black. It 

 has a well-defined posterior margin 

 (Limbus palpebrals posterior), along 

 which the ducts of the tarsal glands 

 open. The anterior margin (Limbus 

 palpebralis anterior) bears stiff hairs 

 termed the cilia (eyelashes) . On the 

 upper lid the cilia are long and num- 

 erous except at its medial third, where 

 they are very small or absent. On 

 the lower lid the cilia are often scarcely 

 distinguishable from the ordinary hairs; 

 in other cases they may be clearly seen 

 except near the lateral canthus, and 

 are much finer and shorter than those 

 of the upper lid. The edge of each lid 

 is pierced near the medial angle by a 

 minute, slit-like opening, the punctum 

 lacrimale, which is the entrance to 

 the lacrimal duct. 

 The skin of the eyelids is thin and freely movable, except near the free edge, 

 where it is more firmly attached. The underlying subcutaneous tissue is destitute 

 of fat. The muscular layer consists chiefly of the elliptical bundles of the orbicu- 

 laris oculi, with which are associated fibers of the corrugator supercilii in the upper 

 lid and the malaris in the lower lid. At the medial side there is a fibrous band, the 

 palpebral ligament, which is attached to the lacrimal tubercle and furnishes origin 

 to some fibers of the orbicularis. At the medial commissure a bundle detached 

 from the orbicularis passes inward behind the lacrimal sac, and is known as the pars 

 lacrimalis (or Horner's muscle). At the lateral side an indistinct palpebral raph6 

 occurs where fibers of the orbicularis decussate. The fibrous layer is thicker and 



'7 3 



Fig. 689. — Vertical Section op Anterior Part of Eye 

 OF Horse, with Lids Half Closed. 

 7, Tarsal gland of upper lid; 2, palpebral conjunctiva; 

 S, fornix conjunctivee; 4i levator palpebrse superioris; 5, 

 orbicularis oculi; 6, cornea; 7, anterior chamber; 5, iris; 

 9, 9', granula iridis; 10, posterior chamber; 11, ciliary proc- 

 ess; 12, ciliary muscle; 13, ciliary zone or suspensory 

 lig. of lens; 14, chorioid; 15, sclera; 16, lens; 17, root of 

 tactile hair. (After Bayer, Augenheilkunde.) 



